Process modeling of electrodes in proton exchange membrane fuel cells

The impedance characteristics of the activation and mass transport overpotentials in a proton exchange membrane fuel cell are determined analytically using a process model. The anode and cathode electrodes are studied separately. The governing equations are solved analytically and the resultant impedances and hence the equivalent circuits are extracted. The results show an excellent agreement between the analytical model and different measured impedances reported in the literature for various operating conditions and anode/cathode feedings. It is shown that the high frequency arc is the result of the anode impedance and is not a function of the current density. On the other hand, the mid-frequency arc consisting of two convoluted depressed semi-circles is the result of the cathode impedance. The solution clarifies the reason for an initial decrease followed by an increase in the diameter of the mid-frequency arc as the current density increases. The results also show that cathode starvation can be studied clearly from the mid frequency arc.